A new fence being built by Hungary to stop migrants from entering the country will be completed by May 1 and a state of emergency giving authorities additional powers to secure the borders has been extended by six months, government officials said.

Prime Minister Viktor Orban's chief of staff, Janos Lazar, said a 10-kilometre (6.2-mile) test section of the second fence on the Serbian border has produced "spectacular" results, with motion sensors and other surveillance tools preventing anyone from breaching it.

Mr Lazar also announced that the "state of emergency due to migration" that has been in place nationwide for the past year will be extended until September 7.

The declaration allows police and the military to be deployed for border defence, among other temporary powers.

"The government believes that there is still a potential threat to Hungary," Mr Lazar said.

"There are about 800,000 migrants stuck in the Balkans who can't go back to Turkey or reach Europe."

The second fence on Hungary's border with Serbia, which some 700 prison inmates started building this week, will be about 150 kilometres (93 miles) long and cost 28.4 billion forints (€92m), he said.

Hungary built fences protected by razor wire on its borders with Serbia and Croatia in 2015, at the height of the migrant flow to western Europe.

Some 400,000 migrants passed through Hungary in 2015 before the fences were erected.

The new fence, as well as proposals to place all asylum seekers in border camps made of shipping containers until their cases are decided, are part of Hungary's new anti-migration efforts.

The new rules for asylum seekers have drawn strong criticism from human rights groups, who say they put protection even further out of reach of people fleeing persecution or war and breach European and international standards.

The expanded border camps in the transit zones will have capacity for 400 people each and will cost a total of 9.4 billion forints (€30.5m), Mr Lazar said.

Hungary has spent 284 billion forints (€921m) so far on the construction and upkeep of border fences, he said.